Action: Protect bird nests using electric fencing

Key messages

One before-and-after study from the UK found an increase in tern numbers after the erection of an electric fence, whilst a study from the USA found an increase in the number of nests.

Five studies from the USA found higher survival or productivity at wader or seabird colonies with electric fencing, compared to areas without fencing, although one study found that hatching rates were no different, whilst nesting success was only higher in one of two years.

One study from the USA found lower predation by mammalian predators inside electric fence exclosures, whilst predation by birds was higher.

Background information and definitions

For fencing to be effective it has to physically prevent predators from entering an area, which is likely to be very difficult for species such as racoons Procyon lotor. Electric fences may offer a solution as animals will be unlikely to attempt to climb through them after receiving electric shocks.

Supporting evidence from individual studies

1

A before-and-after study in 1973 and 1984 on a sand spit in eastern Scotland (Forster 1975) found that the number of sandwich terns Sterna sandvicensis nesting in a colony increased from approximately 80 pairs in 1973 to approximately 450 pairs in 1974, following the erection of a 45 cm high electric fence to separate the colony from the mainland. Previous low numbers were attributed to red fox Vulpes vulpes predation, but after the fence was erected only a single fox was recorded breaching the fence and this animal did not approach the terns.

2

A before-and-after study in 1978 on a beach in Massachusetts, USA (Minsky 1980) found that the number of least tern Sterna antillarum nests in a colony decreased from 138 to 45 between the 20th and 23rd June (red fox Vulpes vulpes tracks were found in the colony), before the erection of an electric fence around the colony on the 24th June. The number of nests increased to 85 following the erection of the fence and no new fox tracks were found within the colony. No nests outside the fence survived. In total, 27 chicks fledged from the colony; the authors estimate that all, or nearly all, came from eggs laid after the erection of the fence.

3

A replicated, controlled trial from 1986-1988 in wetlands in North Dakota, USA (Mayer & Ryan 1991) found that nest survival of 54 piping plover Charadrius melodus nests on four beaches protected by a combined wire mesh and electric fence (1.2 m high, designed to stop mammalian predators) was 71%higher than for 234 nests on 21 unfenced beaches. Chick survival and the fledging rate were 55% and 82% greater on fenced than unfenced beaches, but these increases were not significant.

4

A replicated, controlled trial from 1991-1994 on alkaline flats in Oklahoma, USA (Koenen et al. 1996) found that the nesting success (i.e. at least one egg hatching in a nest) of least terns Sterna antillarum was significantly higher inside two electric fence exclosures than outside (81% of 60 nests vs. 56% of 129 nests respectively). The same pattern was seen for snowy plovers Charadrius alexandrinus, but the difference was not significant (79% of 22 nests vs. 62% of 26 nests). The proportion of both tern and plover eggs predated (mainly by coyotes Canis latrans) was lower inside the fence (10% vs. 20% predation for terns; 6% vs. 11% for plovers. The fence was 86 cm high and designed to prevent coyotes from entering. This study is also discussed in ‘Provide nesting habitat for birds that is safe from extreme weather’.

5

A replicated, controlled trial from 1987-1991 in three wetland-grassland sites in North Dakota and Minnesota, USA (Cowardin et al. 1998) found that using fencing (1.8 m tall with an electrified top wire and with ground-level openings to allow broods to leave) to exclude mammalian predators from 25 ha of nesting habitat significantly increased the nesting success of dabbling ducks Anas spp. (75% of 452 nests inside exclosures), compared to those nesting outside exclosures (no data provided for control). The proportion of nests inside exclosures compared with control areas increased signifcantly for mallard A. platyrhynchos, gadwall A. strepera, blue-winged teal A. discors and northern pintail A. acuta, but not for northern shoveler A. clypeata and dabbling ducks. The authors note that there was a local and regional decline in dabbling duck numbers over the study period, probably due to an ongoing drought.

6

A replicated, controlled trial on the same study site as Koenen et al. (1996) in 1995-6 (Winton et al. 2000) found that the hatching success of snowy plover Charadrius alexandrinus nests was not significantly different (for either year of monitoring) between nests inside three electric fence exclosures (4.5, 24 and 20 ha) and outside exclosures (1995: 44% of nests inside vs. 34% nests outside; 1996: 61% vs. 57%). However, apparent nesting success did differ in 1996 (71% of 17 monitored nests were successful vs. 49% of 160 nests) but not in 1995 (37% of 70 nests inside vs. 38% of 168). The proportion of eggs lost to mammalian predators (mainly coyotes Canis latrans) was lower inside the exclosures (1% vs. 6%), but more eggs were predated by birds, mainly ring-billed gulls Larus delawarensis (11% vs. 3%).

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Effectiveness

An assessment by independent experts of the effectiveness of this action based on the summarized evidence (0% = not effective, 100% = highly effective). This score is based on the direction and size of the effects reported in each study. Actions with high scores typically have large, desirable effects on the target species/habitat in each study. There is some variation between actions, e.g. 100% effectiveness in adding underpasses under roads for bat conservation will likely have different impacts to 100% effectiveness in restoring marsh habitat. The effectiveness score does not consider the quantity or quality of studies; a single, poorly designed study could generate a high effectiveness score. The effectiveness score is combined with the certainty and harms scores to determine the overall effectiveness category (for more details see https://www.conservationevidence.com/content/page/79).

Harms

An assessment by independent experts of the harms of this action to the target group of species/habitat, based on the summarized evidence (0% = none, 100% = major undesirable effects). Undesirable effects on other groups of species/habitats are not considered in this score. The harms score is combined with the effectiveness and certainty scores to determine the overall effectiveness category (for more details see https://www.conservationevidence.com/content/page/79).

Certainty

An assessment by independent experts of the certainty of the evidence for this action based on the summarized evidence (0% = no evidence, 100% = high quality evidence). How certain can we be that the effectiveness score applies to all targets of the intervention (e.g. all birds for an action in the bird synopsis)? This score is based on the number, quality and coverage (species, habitats, geographical locations) of studies. Actions with high scores are supported by lots of well-designed studies with a broad coverage relative to the scope of the intervention. However, the definition of "lots" and "well-designed" will vary between interventions and synopses depending on the breadth of the subject. The certainty score is combined with the effectiveness and harms scores to determine the overall effectiveness category (for more details see https://www.conservationevidence.com/content/page/79).

Overall Effectiveness Category

The overall effectiveness category is determined using effectiveness, certainty and harms scores generated by a structured assessment process with multiple rounds of anonymous scoring and commenting (a modified Delphi method). In this assessment, independent subject experts (listed for each synopsis) interpret the summarized evidence using standardised instructions. For more details see https://www.conservationevidence.com/content/page/79.